1. Field
The present invention relates to communication systems. More particularly, the present invention relates to a system and method for reduction of decoding complexity in a communication system.
2. Background
Communication systems have been developed to allow transmission of information signals from an origination station to a physically distinct destination station. In transmitting information signal from the origination station over a communication channel, the information signal is first converted into a form suitable for efficient transmission over the communication channel. Conversion, or modulation, of the information signal involves varying a parameter of a carrier wave in accordance with the information signal in such a way that the spectrum of the resulting modulated carrier is confined within the communication channel bandwidth. At the destination station the original information signal is replicated from the modulated carrier wave received over the communication channel. Such a replication is generally achieved by using an inverse of the modulation process employed by the origination station.
Modulation also facilitates multiple-access, i.e., simultaneous transmission and/or reception, of several signals over a common communication channel. Multiple-access communication systems often include a plurality of subscriber units requiring intermittent service of relatively short duration rather than continuous access to the common communication channel. Several multiple-access techniques are known in the art, such as time division multiple-access (TDMA), frequency division multiple-access (FDMA), and amplitude modulation multiple-access (AM). Another type of a multiple-access technique is a code division multiple-access (CDMA) spread spectrum system that conforms to the “TIA/EIA/IS-95 Mobile Station-Base Station Compatibility Standard for Dual-Mode Wide-Band Spread Spectrum Cellular System,” hereinafter referred to as the IS-95 standard. The use of CDMA techniques in a multiple-access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE-ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,” and U.S. Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM,” both assigned to the assignee of the present invention.
A multiple-access communication system may be a wireless or wire-line and may carry voice and/or data. An example of a communication system carrying both voice and data is a system in accordance with the IS-95 standard, which specifies transmitting voice and data over the communication channel. A method for transmitting data in code channel frames of fixed size is described in detail in U.S. Pat. No. 5,504,773, entitled “METHOD AND APPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSION”, assigned to the assignee of the present invention. In accordance with the IS-95 standard, the data or voice is partitioned into code channel frames that are 20 milliseconds wide with data rates as high as 14.4 Kbps. Additional examples of a communication systems carrying both voice and data comprise communication systems conforming to the “3rd Generation Partnership Project” (3GPP), embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the W-CDMA standard), or “TR-45.5 Physical Layer Standard for cdma2000 Spread Spectrum Systems” (the IS-2000 standard).
An example of a data only communication system is a high data rate (HDR) communication system that conforms to the TIA/EIA/IS-856 industry standard, hereinafter referred to as the IS-856 standard. This HDR system is based on a communication system disclosed in co-pending application Ser. No. 08/963,386, entitled “METHOD AND APPARATUS FOR HIGH RATE PACKET DATA TRANSMISSION,” filed Nov. 3, 1997, and assigned to the assignee of the present invention. The HDR communication system defines a set of data rates, ranging from 38.4 kbps to 2.4 Mbps, at which an access point (AP) may send data to a subscriber station (access terminal, AT). Because the AP is analogous to a base station, the terminology with respect to cells and sectors is the same as with respect to voice systems.
In a multiple-access communication system, communications between users are conducted through one or more base stations. A first user on one subscriber station communicates to a second user on a second subscriber station by transmitting data on a reverse link to a base station. The base station receives the data and can route the data to another base station. The data is transmitted on a forward link of the same base station, or the other base station, to the second subscriber station. The forward link refers to transmission from a base station to a subscriber station and the reverse link refers to transmission from a subscriber station to a base station. Likewise, the communication can be conducted between a first user on one subscriber station and a second user on a landline station. A base station receives the data from the user on a reverse link, and routes the data through a public switched telephone network (PSTN) to the second user. In many communication systems, e.g., IS-95, W-CDMA, IS-2000, the forward link and the reverse link are allocated separate frequencies.
The above described wireless communication service is an example of a point-to-point communication service. In contrast, broadcast services provide point-to-multipoint communication service. The basic model of a broadcast system consists of a broadcast net of users served by one or more central stations, which transmit information with a certain contents, e.g., news, movies, sports events and the like to the users. Each broadcast net user's subscriber station monitors a common broadcast forward link signal. Because the central station fixedly determines the content, the users are generally not communicating back. Examples of common usage of broadcast services communication systems are TV broadcast, radio broadcast, and the like. Such communication systems are generally highly specialized purpose-build communication systems. With the recent, advancements in wireless cellular telephone systems there has been an interest of utilizing the existing infrastructure of the—mainly point-to-point cellular telephone systems for broadcast services. (As used herein, the term “cellular” systems encompasses communication systems utilizing both cellular and PCS frequencies.)
The information signal to be exchanged among the terminals in a communication system is often organized into a plurality of packets. For the purposes of this description, a packet is a group of bytes, including data (payload) and control elements, arranged into a specific format. The control elements comprise, e.g., a preamble and a quality metric. The quality metric comprises, e.g., cyclical redundancy check (CRC), parity bit(s), and other types of metric known to one skilled in the art. The packets are then formatted to fit into a frame in accordance with a communication channel structure. The frame, appropriately modulated, traveling between the origination terminal and the destination terminal, is affected by characteristics of the communication channel, e.g., signal-to-noise ratio, fading, time variance, and other such characteristics. Such characteristics affect the modulated signal differently in different communication channels. Consequently, transmission of a modulated signal over a wireless communication channel requires different considerations than transmission of a modulated signal over a wire-like communication channel, e.g., a coaxial cable or an optical cable. In addition to selecting modulation appropriate for a particular communication channel, other methods for protecting the information signal have been devised. Such methods comprise, e.g., encoding, symbol repetition, interleaving, and other methods know to one of ordinary skill in the art. However, these methods increase overhead. Therefore, an engineering compromise between reliability of the information signal delivery and the amount of overhead must be made. Even with the above-discussed protection of information signal, the conditions of the communication channel can degrade to the point at which the destination station possibly cannot decode (erases) some of the packets. In data-only communications systems allowing a communication of a feedback from a destination terminal to the origination terminal, one cure is to re-transmit the non-decoded packets using an Automatic Retransmission reQuest (ARQ) made by the destination station to the origination station. However, under certain conditions, the ARQ may overload the communication system. Furthermore, as discussed in regards to broadcast communication systems, the subscribers do not communicate back to the base station. Consequently, other means of information protection are desirable.
A co-pending application Ser. No. 09/933,912, entitled “METHOD AND SYSTEM FOR UTILIZATION OF AN OUTER DECODER IN A BROADCAST SERVICES COMMUNICATION SYSTEM,” filed Aug. 20, 2001, and assigned to the assignee of the present invention, discussed in detail utilization of an outer decoder in a broadcast system. As described in the co-pending application Ser. No. 09/933,912, the bit stream of information to be transmitted is first encoded by an outer decoder and the encoded stream is then encoded by an inner encoder. As illustrated in FIG. 1, the bit stream of information to be transmitted 102, originating at higher layers, is provided to a transmit buffer 104. The transmit buffer is illustrated in more detail in FIG. 2. Referring to FIG. 2, the bits fill the systematic portion 204(1) of the transmit buffer 104 (of FIG. 1) row by row from left to right. The systematic portion 204(1) comprises k rows 208 of length L. Referring back to FIG. 1, once the systematic portion 204(4) (of FIG. 2) is full, the outer block encoder 106 is activated to perform column-wise encoding of the bits in the systematic portion 204(1) (of FIG. 2) to generate (n-k) additional rows 210 (of FIG. 2) of parity bits. This column-wise operation is performed column by column for binary outer code, i.e., m=1. For non-binary code, i.e., m>1, every m adjacent columns in a row are treated as a m-bit symbol. The m-bit symbols along the top k rows are read by the outer encoder to produce n-k m-bit symbols that fill the corresponding lower n-k rows of these columns.
The outer encoder comprises, e.g., a systematic Reed-Solomon (R-S) encoder. Referring back to FIG. 1, the content of the transmit buffer 104 is then provided to a physical layer 108. At the physical layer 108, the individual frames are encoded by an inner encoder (not shown), which results in encoded frames. The structure of the inner decoder may is well known to one of ordinary skills in the art. The systematic rows and the parity rows of the buffer may be interlaced during transmission to reduce the chance of large number of systematic rows erased when the total number of inner code erasure exceeds the outer code's correcting capability. The frames are further processed in accordance with a selected modulation scheme, e.g., cdma2000, WCDMA, UMTS, and other modulation schemes known to one of ordinary skills in the art. The processed frames are then transmitted over a communication channel 110.
The transmitted frames are received at the destination station and provided to a physical layer 112. At the physical layer 112, the individual frames are demodulated and provided to an inner decoder (not shown). The inner decoder decodes each frame, and if the decoding is successful, outputs a correctly decoded frame; or if the decoding is unsuccessful, declares an erasure. The success or failure of decoding must be determined with a high accuracy, achieved e.g., by including a long (for example, 16-bit) cyclic redundancy check (CRC) in the frame after outer encoding and before inner encoding. The included CRC obtained from the decoded frame is compared with a CRC calculated from the bits of the decoded frame, and if the two CRCs are identical, the decoding is declared successful.
If the inner decoder cannot decode the frame, the decoder declares an erasure, and provides an outer block decoder 116 with an indication that the frame is missing. The process continues until there are as many parity frames received correctly and passed to a parity portion 114(2) of a receive buffer 114, as there are erased systematic frames. The receiver stops the reception of any remaining frames and the outer decoder (not shown) is activated to recover the erased systematic frames. The recovered systematic frames are passed to the upper layer.
It is well known in the art that a decoding/error correcting computation complexity increases with increased values of the number of rows in the transmit buffer 104. Because the decoding/error correcting computation complexity affects hardware complexity at the receiving terminal as well as power consumption, there exists a need in the art for a method and system.